With the recent demand to reduce the size and increase the degree of integration of semiconductor devices through the development of semiconductor-manufacturing technology, techniques for forming ultrafine patterns having line widths on the order of tens of nm or less are required. Advances in techniques for forming ultrafine patterns have been made through the use of light sources having smaller wavelengths as well as through the development of processing technology and photoresists suitable for such light sources.
A photoresist is used for photolithography for forming a variety of patterns. The term “photoresist” refers to a mixture of a polymer and a photosensitive agent, in which when the mixture, the chemical properties of which change due to light, is exposed to light at a predetermined wavelength, the solubility thereof in a specific solvent varies, and, due to a difference in dissolution rate between an exposed portion and an unexposed portion of the solvent, the portion that has not been dissolved is left behind after a predetermined dissolution time, thus forming a pattern.
The formation of a photoresist pattern includes negative tone development (NTD) using a negative tone developing solution and positive tone development (PTD) using a positive tone developing solution.
The process of forming a pattern through NTD includes selectively dissolving and removing an unexposed region using a negative tone developing solution, and the process of forming a pattern through PTD includes selectively dissolving and removing an exposed region using a positive tone developing solution.
When compared to pattern formation using PTD, pattern formation using NTD enables the formation of a reverse-phase pattern in a contact-hole pattern or a trench pattern, which is considered to be difficult to form due to insufficient exposure, thereby making it easy to consistently form a pattern. Furthermore, an organic solvent is used as the developing solution for removing the unexposed portion, thereby more effectively forming a photoresist pattern.
Meanwhile, a typical photolithography process using a photoresist composition includes coating a wafer with a photoresist, performing soft baking for heating the coated photoresist to evaporate the solvent, forming an image by means of a light source passed through a photomask, forming a pattern through a difference in solubility between an exposed portion and an unexposed portion using a developing solution, and completing a circuit through etching.
The photoresist composition is composed of a photosensitive agent (a photoacid generator) for generating an acid through excimer laser irradiation, a basic resin and other additives. The basic resin, configured such that a phenol structure contains a hydroxyl group, typically includes a polystyrene polymer, a cresol polymer, or a novolac polymer, but any photosensitive agent may be used so long as it is able to generate an acid (H+) at a specific wavelength, and examples thereof may include sulfonium-, sulfonyl diazo-, benzo sulfonyl-, iodine-, chlorine-, and carboxylic acid-based organic and inorganic acids.
However, a negative photoresist obtained using the above composition is problematic in that the photosensitive agent therein is unable to generate a sufficient amount of acid (H+), making it impossible to form a desired shape, and also in that a profile deteriorates upon the formation of a finer pattern.
Also, a light source mainly used for the above process has a wavelength range of 365 nm to 193 nm, examples of which include an I-line, a KrF excimer laser, and an ArF excimer laser. As is known in the art, the shorter the wavelength, the finer the pattern. In particular, conventional patents for I-line negative photoresist techniques are disclosed in Korean Patent Application Publication No. 2013-0032071 ┌I-line photoresist composition and method of forming fine pattern using the same┐, U.S. Pat. No. 5,627,011 ┌High resolution i-line photoresist of high sensitivity┐, etc.